Manufacturing processes for integrated circuits and liquid crystal display panels typically use optical projection-exposure apparatus to project patterns from a mask onto a substrate. A conventional projection-exposure apparatus 1 is shown in FIG. 9. A light flux L1 from a light source 2 irradiates a mask 4 by means of an optical illumination system 3. The optical illumination system 3 generally comprises a converging lens. The light flux L1 is transmitted by the mask 4 and enters an optical projection system 5 that directs the light flux L1 onto a sensitized surface 7A of a substrate 7. The substrate 7 is held by a Z1-axis stage 6 such that the mask 4 and the substrate 7 are in separate planes parallel to the X1-Y1 plane. The Z1-axis stage 6 adjusts so that the substrate 7 moves along the Z1-axis while remaining in a plane parallel to the plane of the mask 4.
The mask 4 and the Z1-axis stage 6 are fixed to a scan stage 8; the scan stage 8 is held on a stage base 9 so that the scan stage 8 (along with the mask 4 and the Z1-axis stage 6) is movable in the X1-direction and the Y1-direction. In addition, the light source 2, the optical illumination system 3, and the optical projection system 5 are fixed to the stage base 9 by attachment elements not shown in FIG. 9.
A focus sensor 12 comprising a light emitter 10 and a photodetector 11 is placed near the Z1-axis stage 6. The light emitter 10 emits light that is reflected by the sensitized surface 7A to the photodetector 11. A signal output from the focus sensor 12 is used to control the height of the Z1-axis stage 6. By raising or lowering the Z1-axis stage 6, the sensitized surface 7A of the substrate 7 is positioned at the focal plane of the optical projection system 5.
In operation, the scan stage 8 of the projection-exposure apparatus 1 scans at a fixed rate in the X1 and Y1-directions. During this scanning, the focus sensor 12 is used to keep the sensitized surface 7A of the substrate 7 at the focal plane of the optical projection system 5. Thus, the sensitized surface 7A of the substrate 7 is exposed by the light flux L1 to patterns from the mask 4.
The conventional projection-exposure apparatus 1 is difficult to make compact because the Z1-axis stage 6 must adjust the sensitized surface 7A to be at the focal point of the optical projection system 5. In addition, the Z1-axis stage 6 is complex and expensive.
In recent years liquid crystal panels have become widely used in word processors, personal computers, and television. Liquid crystal panels for these applications require transparent thin-film electrodes on large glass substrates; photolithographic processes generally define the shape and location of these electrodes. Therefore, it is necessary that a projection-exposure apparatus implementing the photolithographic processes produce well-formed patterns on large substrates.
A conventional projection-exposure apparatus for large substrates is shown in FIG. 10. As shown in FIG. 10, the projection-exposure apparatus comprises a plurality of optical projection systems 15A, 15B, 15C arranged parallel to a mask 13 and a substrate 14. The plurality of optical projection systems 15A, 15B, 15C exposes a plurality of corresponding regions on the substrate simultaneously. This type of exposure speeds substrate processing.
Because the focal points of the optical projection systems 15A, 15A, 15B, 15C are fixed, it is difficult to focus patterns from the mask 13 at the plurality of regions on the substrate 14, Focus is accomplished by moving a sensitized surface 14A of the substrate 14 to the plane P1 on which the optical projection systems 15A, 15B, 15C are focused. This focus method works when the sensitized surface 14A is flat. When the sensitized surface 14A is not flat, there will be focus errors. Because the optical projection systems 15A, 15B, 15C are fixed with respect to each other, achieving best-focus for a selected optical projection system fixes the focus of the remaining optical projection systems. When the substrate is not flat, the best-focus positions differ for the optical projection systems 15A, 15B, 15C. Such focus errors degrade the resolution with which patterns are transferred. In high-resolution photolithography, even small focus errors are generally unacceptable.
When large substrates are to be exposed in a single exposure, the sensitized surface 14A must be flat. Because keeping a thin substrate flat is difficult, focus errors generally occur. For example, even if the Z1-axis stage 6 (not shown in FIG. 10, but see FIG. 9) is adjusted so that focus is perfect for the light flux light L2A, the images associated with the light fluxes L2B, L2C are not properly focused if the substrate 14 is not flat. Accordingly, lack of substrate flatness causes focus errors.